Low-frequency correcting circuit for wide-band amplifiers



y 1966 J. FOURNOL 3,259,850

LOW-FREQUENCY CORRECTING CIRCUIT FOR WIDE-BAND AMPLIFIERS Original FiledMarch 19, 1963 2 Sheets-Sheet 1 AAAAA F|G.3 FIGA INVENTOR. JACQUESFOURNOL AGE T y 5, 1966 J. FOURNOL 3,259,850

Low-FREQUENCY CORRECTING CIRCUIT FOR WIDE-BAND AMPLIFIERS Original FiledMarch 19. 1963 2 Sheets-Sheet 2 FIG.6

INVENTOR. JACQUES FOURNOL AGEN Claims priority, application UnitedStates Patent 3,259,850 LOW-FREQUENCY CORRECTING CIRCUIT FOR WIDE-BANDAMPLIFIERS Jacques Fournol, Paris, France, assignor to North AmericanPhilips Company, Inc., New York, N.Y., a corporation of DelawareContinuation of application Ser. No. 266,281, Mar. 19, 1963. Thisapplication Aug. 10, 1965, Ser. No. 478,658

France, Apr. 9, 1962, 893,761 (Cl. 330-164) This application is acontinuation of application Serial No. 266,281, filed March 19, 1963,now abandoned.

The invention relates to a low-frequency correcting circuit forwide-band amplifiers and more particularly to correcting circuits fortelevision video amplifiers.

For the amplification of video frequencies resistance amplifier stagesare commonly used which offer the advantage of giving a constant gainfor all the frequencies in a given band.

This advantage is lost, however, when several capacitively coupledamplifier stages are connected in cascade, since the voltage istransferred from the anode of one tube to the grid of the succeedingtube by a shunt circuit comprising the coupling capacitor and theassociated grid leak resistor. The value of the capacitor is limited inorder to avoid the risk of damped or undamped oscillations and the gridresistor is also limited, with the result that the lower the frequency,the more is the gain decreased, this decrease of the gain being 6 db peroctave below the cut-oh frequency of the circuit constituted by thecapacitor and the resistor.

It is known to obviate this disadvantage by connecting the parallelcombination of a resistor and a capacitor in series with the anode loadresistor of the tubes preceding these connections in a manner such thatthe gain at the level of these tubes is inversely proportional to thefrequency so that a variation is produced in a sense opposite to thatproduced by the elements for connection to the succeeding tubes, thusbringing about a compensation for the above described phenomenon. Thiscompensating circuit forms part of every amplifier for the purpose ofdecoupling the high-tension supply and preventing the stages frominfluencing one another through the supply.

In certain cases, however, such a compensating circuit for the very lowfrequencies has a considerable drawback; for optimum response of theamplifier, that is to say, for the voltage drop produced by theconnecting elements to be effectively compensated by the decouplingelements in as much as their effects upon the voltage transferred opposeeach other, it can be proved that the time constant of the circuitcomprising the connecting elements must be equal to the time constant ofthe circuit comprising the decoupling elements and the anode loadresistor.

In a wide-band amplifier the arrangement is made such that the value ofthe anode load resistor is small as compared with the value of thesupply decoupling resistor. The value of the equivalent resistor of thesaid two resistors with respect to the decoupling capacitor canconsequently be reduced to the value of the anode load resistor, and thetime constant of the anode circuit can be considered equal to theproduct of the said load resistor and the capacitance of the capacitor.

A calculation will then show that, with the usual values of the anodeload resistor, the coupling capacitor and the grid leak resistor, thevalue of the decoupling capacitor is very high and such that thiscapacitor is impracticable for comparatively high operating voltageswhen its physical dimensions are to be small.

It is the object of the present invention to provide a low-frequencycompensating circuit for a wide-band amplifier which dispenses with adecoupling capacitor of high value in the anode supply circuit of theamplifier tubes.

arms.

3,259,850 Patented July 5, 1966 The invention is mainly characterised inthat in a resistance amplifier circuit comprising the cascade connectionof at least two capacitively coupled amplifier tubes the anode of eachamplifier tube is fed through an element of variable resistanceconnected in series with the anode load resistor and in which the outputcan be maintained constant, the signal taken from the said anode beingintegrated by means of an adjustable RC-circuit controlling the variableresistance element, while the time constant of this RC-circuit is set toa value such that the resistance variations produced in this manner inthe variable resistance element modify the shape of this signal so as tocompensate for the voltage drop produced by the circuit by which thistube is coupled to the succeeding tube.

The invention is also characterised in that the variable resistanceelement is an auxiliary tube connected as a cathode follower in serieswith the anode load resistor, the control grid of this tube beingconnected through a variable resistance to the anode of the associatedamplifier tube and also through a capacitor to earth, the said couplingresistor and the said capacitor forming the above mentioned adjustableRC-circuit.

The invention may furthermore be characterised in that in an amplifyingcircuit using transistors the variable resistance element is anauxiliary transistor connected in series in the collector circuit ofeach amplifying transistor while each collector is connected through theload resistor of the stage to the emitter of the auxiliary transistorand also through a coupling resistor to the base of this auxiliarytransistor, said base being connected to earth through a capacitor,while said coupling resistor and said capacitor form the above mentionedRC-circuit.

It should be noted that in the last mentioned circuit arrangement theoutput electrode of the transistor of a stage is connected to the baseof the auxiliary transistor through the variable resistor which togetherwith the said capacitor forms the RC-integrating circuit for thecompensating signal. In this case a calculation shows that this resistorcan be considered as a means for adjusting the value of the compensationintroduced into the circuit by influencing the bias of the auxiliarytransistor.

In a modified embodiment of the invention, if the amplifier istransistorized and the compensation is not exact, the collector of eachamplifying transistor is connected through the load resistor of thestage to the emitter of the auxiliary transistor and also directly tothe base of this auxiliary transistor, the impedance, offered by thistransistor in series with the impedance of the said capacitordetermining the value of the resulting compensation.

In order that the invention may readily be carried into effect, anembodiment thereof will now be described, by way of example, withreference to the accompanying diagrammatic drawings, in which: I

FIG. '1 shows the interstage circuit of a tube amplifier provided withcompensation by a known method, while FIGS. 2 and 3 both are equivalentdiagrams illustrating the principle on which the invention is based;

FIG. 4 is a schematic diagram of the correcting circuit in accordancewith the invention using tubes;

FIG. 5 is a schematic diagram of the correcting circuit in accordancewith the invention using transistors, and

FIG. 6 is a practical circuit diagram of part of a video amplifierincluding a correcting circuit.

FIG. 1 shows partly two tubes T1 and T2, the anode circuit of the tubeT1 comprising a load resistor Ra, a decoupling resistor Rd and adecoupling capacitor Cd.

, The connection to the grid of the tube T2, which grid is connected toearth through a grid leak resistor Rg, includes a coupling capacitor Cl.

In an amplifier provided with resistance-capacity couplinig, part ofwhich is shown in FIG. 1, the value of the grid leak resistor Rg, whichin principle-is comparatively large, should not exceed a certain(limiting value in Order not to give rise to grid currents which in turnmay give rise to relaxation oscillations. Generally the value 'of theresistor Rg is limited to 1M ohm.

For reasons of physical dimensions and also to prevent the time constantof the circuit Cl-Rg from becoming unduly large and giving rise torelaxation oscillations,

the value of the capacitor Cl is limited to a few tenths of amicrofarad.

However, a consideration of the circuit comprising Cl and Rg whichcouples the tubes T1 and T2 to each other shows that the alternatingvoltage taken from the tenninals of the anode circuit of T1 iseffectively transferred 'to the grid of T2 by. a dividing bridge in theratio 'inbefore the time constant, that is to say the values of kg andCl, cannot be increased beyond certain limits, a

considerable voltage loss must be accepted for the low frequencies, andthe decrease in gain may reach 6 db per octave below the cut-offfrequency of the circuit comprisirig Rg and Cl.

It is known to obviate this disadvantage by inserting into the anodecircuit of the tube T1 a capacitor-resistor circuit in series with theload resistor, this circuit being designated in FIG. 1 by Cd and Rd,respectively. At the low frequencies such a circuit, which promotes thegain of the stage constituted by T2, permits of compensating inprinciple for the attenuation produced by the coupling circuits 'D1Rg.The gain of the stage can be written:

Rd 1+ jRdCdw) where Za is the impedance of the anode load circuit and wis the angular frequency of the transmitted signal.

This shows that the gain of the stage is higher in proportion as thefrequency of the transmitted signal is lower, and this provides inprinciple the above mentioned compensation.

In order to obtain complete compensation the over-all gain must berendered independent of the frequency that is to say the values of theelements Ra, Rd, Cd must be chosen as a function of the values of Rg andCl so that the terms in to cancel each other in the expression for thisgain, that is to say, by combining (1) and (2).

Rd 1 I G 1+jRdC'dw) (1+l/jRgC'lw) The second factor of the second memberof the expression, that is to say Za, can be transformed:

In the case-under consideration a wide-band amplifier is concerned, thatis to say, an amplifier capable of passing signals the frequency ofwhich may vary between a few cycles per second and several megacyclesper second.

In such an amplifier the stray capacitances (cabling capacitance,capacitance between the anode and the electrodes of T1, grid-cathodecapacitance of T2) assume such values at the high frequencies that theyform effective short-circuits therefor when the load resistor Ra has ahigh value. Hence this latter value must be reduced to an extent suchthat at the high frequencies it is at least comparable to the impedanceformed by the said stray capacitances.

In practice, in a video amplifier for a flying-spot generator we have,for example, the following values:

Ra: 1.3Ktl

and

Rd=15Kfl In certain cases, Ra may even be smaller with respect to Rd: asan example we may mention a stage for compensating for the imageretention of the flying-spot tube, in which Ra=tz and Rd= l5KtL Hence wemay generally write:

Ra Rd Under these circumstances, when we return to the expression (3)Ra-l-Rd RaRd 1+ Rdcdw Ra+Rd we may write m: Ra+Rd Rd 1 1 1+ RdCdw l-l-RdCdw 1 jcdw (since Rd is large, l/Rd is small) and n=1+jRaCdw Theexpression Za then becomes:

1 were If 'noiv in the expression for G the value found (4) issubstituted for Za, we find G 1+1/jRgC'lw To render the arrangementindependent of the frequency, that is to say of the angular frequencyto, We must have the following equation RaCd=RgCl In the case of theimage retention correcting amplifier mentioned hereinbefore, whereRd=15Kfl Ra Ra: 1109 if we take the commonly used values Cl=0.1;tf.Rg=1MQ then the capacitor 4. This circuit utilizes the property of acathode follower stage that at the cathode side it offers a generallylow impedance, the anode of the amplifier tube T1 being fed 1 throughthe tube t.

The cathode load circuit of the tube t actually consists of a resistanceof high value (in practice Rg and the inter, nal resistance of theamplifier tube in parallel).

The anode and grid voltages of t will be assumed constant. Theequivalent diagram for variable operation then is shown in FIG. 2. It isassumed that p=the internal resistance of the tube t, S=its slope,,u.=its amplification factor,

Ia=the current flow through it, Vc=the alternating voltage set up at theterminals of Re, the equivalent load resistance of the cathode circuit,Z=an impedance of arbitrary value inserted in the anode circuit.

We can write p.VC-VC from which can be deduced the value of theresistance viewed from the cathode When a screen-grid tube having a highinternal resistance, a large amplification factor and a steep slope, forexample, a tube of the type EF42 (where 70000 and ,u#80), is connectedas a triode and the impedance Z is low, Z can be neglected with respectto p and 1 can be neglected with respect to [J- and hence:

p l S Consequently, if the slope is steep, r is so small that the tube 1acts as a pure resistance of low value of which the base is completelydecoupled, so that the equivalent diagram of the anode load resistor ofthe tube T1 is that shown in FIG. 3.

It will be noted, however, that under the mere conditions that r issmall and Ra cannot be large for the reason set forth hereinbefore, thevalue of G is still high and prohibitive if the above mentionedrelationship and Yiul l'( a) la 19 Ia 1+ 1 If ,u. is large enough, wemay write:

l K9 E s Ia and the impedance z in the anode of the amplifier, that isto say Vc/Ia is equal to 1/S-Vg/Ia. For an effective compensation of thelow frequencies this impedance must be reduced, as has been mentionedhereinbefore, to the form z=A-j/Gw or, by combining the two equations,

l Z2 TE S+ 10 Vg=Ia/Gw. But it is still possible to write mIa m1 a 1nGwCgw where m is as small as may be desired and Cg constitutes the gridload of the tube 1 traversed by the current Ia. mIa can be obtained bytaking part of the amplified voltage from the anode of the amplifyingtube T1 through a large resistance.

This results in the circuit diagram shown in FIG. 4, in which thecompensating tube t is connected as a cathode follower in the anode loadcircuit of the tube T1, while the grid load of the tube 1 is constitutedby the capacitor Cg traversed by the anode current owing to the presenceof the resistance R which in this case is calculated in the followingmanner: We may write:

1 J' Ia[(Ra+r) ]#(R mIa where r is approximately equal to US (where S isthe slope of the tube 2?), G is the theoretical value of the capacitancerequired for the compensation, m being such that mG, that is to say mCg,has a value which is acceptable in practice.

From this equation we have FIG. 5 shows, by way of example, theschematic diagram of part of a compensating circuit in accordance withthe invention which, however, is equipped with transistors. The sameelements are used and their functions consequently need not beexplained.

Finally FIG. 6 shows the practicaland detailed circuit diagram of avideo amplifier equipped with tubes and provided with a correctingcircuit as shown in FIG. 4.

The tube t is a pentode tube connected as a triode, the anode load ofwhich mainly consists of the capacitor Cz which has a smalllow-frequency impedance. It is connected in the anode load circuit ofthe amplifier tube T1 in series with the resistance Ra and a correctingcoil L1 which is conventionally used in circuits of this type.

The anode current flows through the grid load of the tube t, which loadconsists of the capacitor Cg, byway of a resistor R and a potentiometerR". This potentiometer permits of adjusting the introduced compensation.

In a practice a square signal having a very low frequency is applied atE to the grid of the tube T1. While observing the signal taken from thegrid of the stage consisting of the tube T2, the potentiometer R" isadjusted so that the porches appear as horizontal as possible to theeye.

With the values given hereinafter adjustment may be effected Withoutvisible distortion of the rectangular signal when its period is sec.

The fixed bias of the grid of the tube t is taken from a voltage dividerR2-R3.

The cathode-grid circuits of the tube T1 are of the conventional typeand its connection to the succeeding tube, which connection comprisesthe capacitor Cl and the grid leak resistor Rg, also includes aconventional correcting coil L2. The coils L1 and L2 provide a mixedseries-parallel compensation for the high frequencies.

The values of the elements which are characteristic of the circuit arethe following:

T1, T2, 2 Tubes of the type EF42. R 390KQ.

Ra 1.3KS2.

Cl 0.25m.

Rg 1M9.

Obviously the circuit shown in FIG. 6 may be equipped with transistorsby taking into consideration the modifications of values and-connectionsinherent in the use of these elements, while the principle of thecompensation, that is to say, the principle of the invention, remainsexactly the same. It will be appreciated that the preceding descriptionhas been given by way of non-limitative example and that modificationsespecially with respect to the form of the circuit when it is equippedwith transistors (paricularly, the resistor R can be omitted if no exactcompensation is wanted), may be performed without departing from thescope of the invention.

What is claimed is:

1. A Wide band amplifying circuit comprising; a first amplifying stagefor receiving the signal to be amplified; a second amplifying stage;first capacitive means connecting the output of the first stage with theinput of the second stage; impedance means connecting the input of saidsecond stage to a source of reference potential; load means connected tothe output of said first stage; a third amplifying stage including acontrol electrode, said third amplifying stage being connected betweensaid load means and a source of constant potential; and secondcapacitive means connecting said control electrode to said source ofreference potential whereby the impedance of said third stage varies asa function of signal frequency to provide a flat frequencycharacteristic for the amplifier circuit over a wide frequency band.

2. A Wide band amplifying circuit comprising; a first amplifying stagefor receiving the signal to be amplified; a second amplifying stage;firstcapacitive means connecting the output of the first stage with theinput of the second stage; impedance means connecting the input of saidsecond stage to a source of reference potential; load means connected tothe output of said first stage; a third amplifying stage including acontrol electrode, said third amplifying stage being connected betweensaid load means and a source of constant potential; means connecting theoutput of said first amplifying stage to said control electrode; andsecond capacitive means connecting said control electrode to said sourceof referencepotential whereby the impedance of said third stage variesas a function of signal frequency to provide a flat frequencycharacteristic for the amplifier circuit over a wide frequency band.

3. A wide band amplifying circuit comprising; a first vacuum tubeamplifier stage having cathode, gridand,

plate electrodes; a second vacuum tube amplifier, stage including acontrol grid electrode; capacitive coupling means interconnecting theplate electrode of said first stage and the control grid electrode ofsaid second stage;

impedance means coupling the control grid of said sec- 0nd stage to asource of reference potential; load circuit means connected to the plateelectrode of said first stage;

a third vacuum tube amplifier stage having a cathode connected to saidload circuit means, a plate electrode connected to a first source ofsupply voltage and a control grid electrode connected to the plateelectrode of said first stage by resistive means and to said source ofreference potential by capacitive means whereby the impedance betweenthe cathode and plate electrodes of said third stage varies as afunction of the signal frequency applied to said first stage to providea. flat frequency char- 7 acteristic for the amplifier circuit over awide frequency band.

4. A wide band amplifying circuit comprising; a first,

transistor amplifier stage having a common, input and output electrode;a second transistor amplifier stage in cluding an input electrode;capacitive coupling means interconnecting said output electrode of saidfirst stage and the input electrode of said second stage; impedancemeans coupling the input electrode of said second stage to a source ofreference potential; load circuit means connected to said outputelectrode of said first stage; a third transistor amplifier stage havingauemitter electrode connected to said load circuit means, a collectorelectrode connected to a first source of supply voltage and a baseelectrode connected by resistive means to the output electrode of saidfirst stage and to said source of reference potential by capacitivemeans whereby the impedance between the emitter and collector of saidthird stage varies as a function signal frequency applied to said firststage input electrode to provide a flat frequency characteristic for theamplifier circuit over a wide frequency band.

References, Cited by the Examiner,

UNITED STATES PATENTS 3,024,423 3/1962 Azelickis et al. 330-71 FOREIGNPATENTS 954,531 6/1949 France.

ROY LAKE, Primary Examiner. NATHAN KAUFMAN, Examiner.

1. A WIDE BAND AMPLIFYING CIRCUIT COMPRISING; A FIRST AMPLIFYING STAGEFOR RECEIVING THE SIGNAL TO BE AMPLIFIED; A SECOND AMPLIFYING STAGE;FIRST CAPACTIVE MEANS CONNECTING THE OUTPUT OF THE FIRST STAGE WITHINPUT OF THE SECOND STAGE; IMPEDANCE MEANS CONNECTING THE INPUT OF SAIDSECOND STAGE TO A SOURCE OF REFERENCE POTENTIAL; LOAD MEANS CONNECTED TOTHE OUTPUT OF SAID FIRST STAGE; A THIRD AMPLIFYING STAGE INCLUDING ACONTROL ELECTRODE, SAID THIRD AMPLIFYING STAGE BEING CONNECTED BETWEENSAID LOAD MEANS AND A SOURCE OF CONSTANT POTENTIAL; AND SECONDCAPACITIVE MEANS CONNECTING SAID CONTROL ELECTRODE TO SAID SOURCE OFREFERENCE POTENTIAL WHEREBY THE IMPEDANCE OF SAID THIRD STAGE VARIES ASA FUNCTION OF SIGNAL FREQUENCY TO PROVIDE A FLAT FREQUENCYCHARACTERISTIC FOR THE AMPLIFIER CIRCUIT OVER A WIDE FREQUENCY BAND.